Mechanical pulping is a process of mechanically triturating wood into fibers for the purpose of making pulp. Mechanical pulping is attractive as a method for pulping because it achieves higher yields as compared with chemical pulping since lignin is retained to a large degree in mechanically pulped woods. Pulps made using any of the conventional mechanical pulping methods are mainly used for newsprint and printing papers but are typically unsuitable for high quality or durable paper products. This is due, in part, to the fact that high yield mechanical pulps are generally more difficult to bleach than chemical pulps because of the high lignin content.
There are many types of mechanical pulping, including stone grinding (SG), pressurized stone grinding (PSG), refiner mechanical pulping (RMP), thermomechanical pulping (TMP), and chemi-thermomechanical pulping (CTMP). The latter three can further be grouped generally under refiner pulping processes. In RMP, wood chips are ground between rotating metal disks. The process usually is carried out in two stages. The first stage is mainly used to separate the fibers, while the second stage is used to treat the fiber surface for improved fiber bonding of paper products. In RMP, the wood chips are refined at atmospheric pressure in both a first and a second stage refiner. The refiner processes generate heat by the friction of the metal disks rubbing against the wood. The heat is liberated as steam, which is often used to soften the incoming chips.
TMP differs from RMP in that the pulp is processed in a pressurized refiner. In the TMP process, two stages are normally used also. The first stage refiner operates at an elevated temperature and pressure, and the second stage refiner is typically at or near atmospheric pressure. Pulps made by a TMP process have high strength, which makes the TMP process the most favored mechanical pulping process. However, there is still room for improving the TMP process. The TMP process consumes large amounts of energy, and the pulp produced by the TMP process tends to be darker than most other pulps. Alkaline bleaching of mechanical pulps produced by the TMP process has been carried out using oxidative reagents, such as hydrogen peroxide. Sodium hydroxide is a strong alkali that provides the requisite high pH necessary to produce the active perhydroxyl ion, HOO−, thought to be the agent primarily responsible for bleaching.
U.S. Pat. No. 4,270,976 to Sandstrom et al., is representative of a TMP process used to produce peroxide bleached, mechanical pulp by introducing a peroxide containing bleaching solution into the grinding space of a refiner. The conventional alkalinity in the Sandstrom patent is supplied by caustic (sodium hydroxide). Sodium hydroxide requires the use of sodium silicate, which 1) acts as a pH buffer for the sodium hydroxide and 2) helps in stabilizing the peroxide. The peroxide bleaching causes oxalate formation. The highly dissolved alkali concentration with sodium hydroxide and sodium silicate promotes oxalate scale deposits on the refiner plates, interfering with the operation and efficiency of the refiner. Oxalate scale can even be present in the finished paper products. Refiner bleaching using sodium hydroxide and sodium silicate causes refiner plate filling, erratic refiner load, and “slick” pulp resulting in inadequate refining of the wood. The use of sodium silicate also requires separate facilities to store the chemical and pumps to meter the correct dosage. Darkening of the pulp can be attributed to the addition of excess quantities of sodium hydroxide. The aforementioned problems illustrate that refiner bleaching with sodium hydroxide and sodium silicate has many drawbacks that make commercial use difficult and expensive.
Accordingly, there is a need to find alternative methods of refiner bleaching that cures many of the aforementioned problems with using sodium hydroxide and sodium silicate.
The prior U.S. application Ser. No. 09/860,025, filed May 16, 2001, incorporated herein by reference in its entirety, and assigned to the assignee of the present application, describes using substitute alkaline chemicals for sodium hydroxide. The present application further adds to the methods of the '025 application.